A lithographic master is prepared by coating a flexible metal plate with a light-sensitive composition. A stencil bearing the desired graphic intelligence is then locked over the coated plate in a vacuum printing frame to ensure perfect contact between stencil and plate. The image is then "burned in" as with actinic radiation which shines down through the stencil to the plate, hardening the composition where irradiated and rendering it insoluble in water. Nonprinting areas, which are shielded by the opaque parts of the stencil, are not affected by the action of the light and remain soluble.
After exposure, which varies in length of time according to the nature of the work to be printed, the stencil and plate are removed from the printing frame and separated, the face of the plate is coated with ink, and the plate is then washed. The ink adheres to and brings out the printing image but washes away from the nonprinting areas, carrying the composition with it and leaving the metal exposed. The printing image is then fixed, and subject to final change or correction, the plate is ready for the press.